Trump says US-Russia relations may be at "all-time low"

Pressed by reporters on the future of Assad, he added: "In terms of the future of Bashar al-Assad, it is important to us that we undertake a political process that leads to the final conclusion of how Syria will be governed".

It's unclear, Tillerson noted, whether Moscow had failed to take seriously its responsibility to purge Syria of chemical weapons or was just incompetent.

"Young children dying, babies dying, fathers holding children in their arms that were dead, dead children", Trump said.

He says Russian Federation must choose between aligning itself with the US and like-minded countries or embracing Syrian President Bashar al-Assad, Iran and the Islamic radical group Hezbollah.

The two leaders were expected to discuss disagreement about whether Syrian President Bashar Assad or his opposition was responsible for a chemical weapons attack last week that led to United States retaliatory cruise missiles. He says Russian Federation can play a role in Syria's future but that aligning with Assad won't serve Russia's long-term interests.

"It has become even more hard for Trump, but it is now even more hard for Putin as well, because he has to show he is a strong leader and that he will not yield under the USA pressure", Russian International Affairs Council Director Andrey Kortunov told CBS News.

Asked about Putin possibly skipping a meeting with Tillerson, White House press secretary Sean Spicer said, "There is a bit of irony that for all of these talks that have been perpetuated about back channels and direct links, that now it's they won't meet with you".

The US attack in Syria has nothing to do with attempts to uncover the truth about the use of chemical weapons in Idlib province, according to Russian Foreign Ministry Spokeswoman Maria Zakharova.

A No 10 spokesman said: "The Prime Minister and the president agreed that a window opportunity now exists in which to persuade Russian Federation that its alliance with Assad is no longer in its strategic interest".

"Now I think is an opportunity for Russian Federation to make that break and to use its influence with Assad to push the Assad regime to fairly participate in political negotiations that can bring peace to Syria".

"I do not believe that the Russians want to have a worsening relationship with the U.S.", Tillerson said in a TV interview on Sunday.

Tillerson has a meeting set later Wednesday in Moscow with Foreign Minister Sergey Lavrov. Yet the US has no proof of Moscow's involvement, said the official, who wasn't authorized to speak publicly on intelligence matters and demanded anonymity.